1. Field of the Invention
The present invention relates to a charged particle beam apparatus.
2. Description of the Related Art
A scanning electron microscope as one of the charged particle beam apparatuses is an apparatus which accelerates electrons emitted from a heating type or field emission type of electron source to form a fine electron beam (primary electron beam) using an electrostatic lens or an electromagnetic lens, two-dimensionally scans a sample to be observed using the primary electron beam, detects secondary particles such as secondary electrons or backscattered electrons (reflected electrons) secondarily generated from the sample when being irradiated with the primary electron beam, sets a detected signal intensity to a brightness modulation input of a display device such as a cathode ray tube which performs scanning in synchronization with the scanning of the primary electron beam, and thereby obtains a two-dimensional scanning image.
In a process of manufacturing a semiconductor device, there is a need to observe a situation of defects generated by a failure of the manufacturing process in an in-line environment and to clarify and feedback the cause to the manufacturing process in order to realize an improvement in yield and to stabilize the manufacturing process. The observation of a fine defect is one of principal techniques required for managing the process, and an optical microscope has been used so far to evaluate the defect. However, nowadays a pattern size of the semiconductor device has been minimized, and accordingly sizes of defects adversely affecting on the device properties have been minimized. Therefore, the evaluation using the scanning electron microscope becomes important. Therefore, a scanning electron microscope has been commercialized with which a defect can be reviewed with a high resolution. In the apparatus, it is important to acquire a shaded image of the scanning electron microscope image equivalent to the shading generated when the sample is irradiated with the light from the lateral side in order to detect irregularities such as a fine foreign material or scratches.
A basic principle of acquiring the shaded image using the scanning electron microscope will be described using FIGS. 2A and 2B. Herein, the description will be exemplarily given about a dip of the sample in which a signal profile obtained by detectors provided on either side is changed when an electronic beam is emitted. In FIG. 2A, a detector 33 is provided on the left side of the dip. When a primary electron beam 32 is emitted, secondary electrons 34 emitted in the direction toward the detector 33 are detected. However, on the left side in the dip, some secondary electrons 35 come into conflict with the wall surface of the dip before the particles reach the detector 33, and thus cannot be detected. A signal profile 36 obtained at this time is observed such that the left side of the dip is dark and the right side is bright. On the other hand, in a case where a detector 37 is provided on the right side of the dip, secondary electrons 38 emitted in the direction toward the detector 37 are detected. However, some secondary electrons 39 generated on the right side in the dip come into conflict with the wall surface of the dip, and thus not detected. On the contrary to the above description, a signal profile 40 obtained at this time is obtained such that the right side of the dip is dark and the left side is bright. Therefore, in a case where the observation target is recessed, a portion of the observation target near the detector becomes dark, and the opposite side becomes bright. On the other hand, in a case where the observation target is projected, a portion of the observation target near the detector becomes bright, and the opposite side becomes dark. In this way, it is possible to determine whether the shape of the observation target sample is projected or recessed by inspecting a relation of the layout of the detectors and the brightness and darkness of the signal profile.
Even when there is one detector, the irregularity information on one direction of the sample is obtained from the brightness and darkness appearing in the image of the secondary electrons formed using the detector. However, the irregularity information on one direction of the sample can be obtained with more accuracy when a pair of detectors is disposed symmetrically to an optical axis. Further, when a plurality of pairs of detectors are disposed symmetrically to the optical axis, the irregularity information on the surface of the sample can be obtained from a plurality of directions.
A technique of obtaining the irregularity information on the sample on the basis of such a principle is disclosed in JP 2001-110351 A and JP 8-273569 A. In JP 2001-110351 A, an objective lens of a semi-in-lens type is used to achieve a high resolution, the detector is disposed on a side near an electron source of the objective lens, and the electric field between the sample and the objective lens is controlled, so that the reflected electrons emitted in a direction forming a small angle with respect to the surface of the sample are detected and the irregularities of the sample are determined. In JP 8-273569 A, annular detectors are used, and the reflected electrons are detected by an inside annular band, and the secondary electrons are detected by an outside annular band. Then, the outside annular band is divided into four blade parts, a shaded image is obtained by selecting an azimuth angle for emitting the secondary electrons, and a profile of the sample piece and a characteristic of the material are measured. JP 2013-2872 A relates to a technique of quantitatively analyzing a three-dimensional shape of the observation target, in which four independent detectors are provided symmetrically to the optical axis, and the signal profiles obtained by the respective detectors are assembled and analyzed so as to measure a width and a slope angle of an edge of a line pattern.